Retrofitting an ERV
Hi all,
Can I unbalance a balanced ERV for make-up air purposes?
I’m planning on installing Zehnder ERVs with dedicated ducting in a new build.
Realizing that these systems are supposed to provide balanced ventilation, even in boost mode, is it possible to set up custom modes that are intentionally imbalanced? Specifically I’m thinking that it would be nice to have the ERV flip to a custom mode where it sucks in an extra 60-100 CFM every time the exhaust fans in the bathrooms come on or if/when the dryer is running.
(I’m planning on having dedicated make-up air for the kitchen exhaust but would like to avoid makeup air otherwise).
Has this been done? Does it work? Any unforeseen consequences that may arise?
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The hardware is capable of doing that, and you can go into the menu and set up an imbalance in airflow, or even turn off the exhaust. But you won't want to dig through the menu each time you turn on the bath fan, so the question is whether the controls are set up to allow you to do that with an external signal. You probably need to ask Zehnder about that.
But I will say that if you get one big enough, using the boost mode, which can be activated with their standard wireless remote, is an option for for bathrooms exhaust that has worked out well for me.
Thanks. Very helpful. I was under the impression that there were pre-set modes on the Zehnder that you can customize, but I guess I'll have to reach out to to them to confirm the controls question.
I'm trying to figure out whether or not I want direct venting bath fans, or have them connect to the return side of the Zehnder.
One consideration I have is that we're considering a steam shower in the master bath, so for that bathroom at a minimum, our engineer thinks we'll need to direct vent. I don't want to go down the heat pump dryer path, so that will also have a vent.
And then at that point, just thinking about the other baths in the house, not really sure what approach is best. We are planning on having dedicated (separate) dehumidification in the house. Perhaps that means having the bathrooms return to the Zehnder could be feasible?
The firs question is if you actually need to provide makeup for the bath and dryer. Unless you have a combustion appliance, depressurizing the house a bit is not a problem. It takes a pretty tight or small house for it to be depressurized a significant amount by a bath fan or even a dryer. If needed for the dryer, you can always interlock it with a current sensing relay to the MUA of the range.
There is nothing wrong with using the ERV for exhausting a bath, even a steam shower. As long as you have the right flow rate, it will work. The ERV will recover some of the moisture from the shower but these are short enough compared to a full day that it simply doesn't matter.
Is there any way to measure this? Essentially energy modelling -- but for humidity instead of heat gain/loss? Is there a specific metric I should ask my engineer about to be able to verify that we're set up to manage the humidity in the house.
I would prefer to be able to rely on the ERV + dedicated dehu to handle the bathrooms and dryer and just generally limit the number of penetrations through the envelope. But I know that my wife and kids will generally be pretty upset if the bathrooms are fogged up and clammy for multiple hours after every shower.
Also, separately but somewhat relatedly, while we're on the topic, if I'm going to have a Zehnder unit with its own duct system for ventilation, and am going to use a series of ducted but zoned mini splits for heating and cooling, what should I do with the dehumidifier? Should that be installed somehow inline and using the same duct network as the Zehnder? Should it have its own dedicated duct network? What is best practice for this type of thing if you don't have a single central air handler for heating/cooling purposes?
Take a look at #4 here:
https://www.greenbuildingadvisor.com/question/question-regarding-erv-to-vent-bathroom
TLDR; some humidity is reintroduced into the house but it is minor.
There are no issues with an ERV clearing a bathroom provided the airflow is sufficient. A stale air pickup with above 50cfm on boost is good enough. You might want to bump that up a bit with a steam shower. I have both ERV and bath exhaust fan in mine, the ERV (33cfm) is good enough for showers but the exhaust fan is needed after chili night.
Dryers have nothing to do with ERV or dehumidifier. A heap pump unit doesn't put moisture into the house and a standard vented one send the moisture outside.
Since you have multiple ducted units you might not need a dedicated dehumidifier. I've had good luck with removing construction moisture from drywalling by running the 3rd floor ducted unit in dry mode and the 2nd floor unit on heat. The extra bit of heat from the 2nd floor unit keeps the one on dry from overcooling the house and it doesn't cost all that much in power usage. Definitely less than running stand alone box store dehumidifier + AC. This assumes each ducted unit is on its own outdoor unit which it should be.
If you do end up installing a stand alone dehumidifier, these are typically installed and vented into the basement/crawlspace. The extra bit of heat from the dehumidifier helps keep a basement from feeling cold and clammy in the summer time. Moisture will move quite freely through the house, provided your place is air tight, dewpoint will be the same no matter where the dehumidifier is located.